(i) Technical Field
A certain aspect of the embodiments discussed herein is related to a semiconductor substrate and a semiconductor device. Another aspect of the embodiments is related to a semiconductor substrate and a semiconductor device in which an aluminum nitride (AlN) layer is formed on a silicon (Si) substrate.
(ii) Related Art
Semiconductor devices using a gallium nitride (GaN) based semiconductor are used as a power device operating at high frequencies and outputting high power, and a light emitting diode or a laser diode emitting light of short wavelengths. Particularly, there has been considerable activity in the development of semiconductor devices suitable for amplification in a high-frequency or RF (radio Frequency) band such as a microwave band, a quasi-millimeter band or a millimeter band. Examples of those semiconductor devices are an FET such as a high electron mobility transistor (HEMT) and light emitting devices such as a laser diode and a light emitting diode.
Generally, a sapphire substrate or a silicon carbide (SiC) substrate is used as a substrate on which the GaN-based semiconductor layer is grown. Since the sapphire substrate and the SiC substrate are expensive, there has been developed an art of growing the GaN-based semiconductor layer on the Si substrate. As Si and Ga react easily, an AlN layer is interposed between the Si substrate and the GaN-based semiconductor layer as a barrier layer (see Japanese Patent Application Publication No. 2008-166349). It is known to form an AlGaN layer between the AlN layer and the GaN layer when the GaN layer is formed on the AlN layer. The use of the AlGaN layer is intended to improve the crystal quality of the GaN layer and reduce a warp of the substrate.
However, there is a case where the warp of the substrate is not reduced even by providing the AlGaN layer on AlN layer.